The present invention relates to a catalyst composition for purification of exhaust gas, a catalyst for purification of exhaust gas and a process for producing the catalyst. The present invention relates mainly to a three-way catalyst for purification of exhaust gas emitted from internal combustion engines of automobiles.
Various types of catalysts for purification of exhaust gas emitted from internal combustion engines have hitherto been proposed. For example, Japanese Patent Application Laid-Open No. 135541/1989 proposes a zeolite catalyst subjected to ion exchange with a noble metal such as Pt, Pd - - - , - - - ; for purification of NO.sub.x at a high conversion efficiency even for an exhaust gas generated under a large air-to-fuel ratio (A/F) (a fuel-lean mixture). Japanese Patent Application Laid-Open No. 139144/1989 proposes, also as a catalyst for purification of exhaust gas emitted from lean burn engines, a catalyst which is a mixture of (1) a reduction catalyst wherein zeolite is subjected to ion exchange with a transition metal and (2) an oxidation catalyst comprising alumina and a noble metal catalyst component supported thereon.
Japanese Patent Application Laid-Open No. 56247/1990 proposes, for purification of hydrocarbons at a high conversion efficiency particularly at cold start, a three-way catalyst consisting of a first layer comprising, as a main component, zeolite supported on a carrier and a second layer comprising, as a main component, a noble metal such as Pt, Pd, Rh. Japanese Patent Application Laid-Open No. 174937/1990 proposes a three-way catalyst consisting of molecular sieves and a noble metal, developed for the purpose of reducing the amount of expensive Rh.
However, in the catalysts proposed in Japanese Patent Application Laid-Open Nos. 135541/1989, 139144/1989 and 56247/1990 all employing, as the catalyst component, a noble metal supported on zeolite, no study has been made on the effect which Si/Al ratio of zeolite has on catalytic activity; therefore, some of those catalysts have insufficient heat resistance and do not function sufficiently as a catalyst for exhaust gas purification. The activity depends upon the level of Si/Al ratio. Moreover, in the catalyst proposed by Japanese Patent Application Laid-Open No. 56247/1990 wherein hydrocarbons are adsorbed by the inner zeolite layer at cold start and, after warm-up, are desorbed, oxidized and thereby made nontoxic, the outer layer becomes a hindrance to the mass transfer of substances, making it difficult to obtain an intended effect sufficiently.
In the catalyst proposed by Japanese Patent Application Laid-Open No. 174937/1990 employing various zeolites as a three-way catalyst, the noble metal is not sufficiently supported on any of these zeolites, making it impossible to obtain a sufficient catalytic activity for exhaust gas purification.